This invention relates to thermal-radiation detectors comprising a group of infrared detector elements of ferroelectric and/or pyroelectric material, and to detection systems comprising such detectors and modulation means (for example, a chopper) in front of the detector elements for periodically modulating the transmission of the thermal-radiation to the detector elements. Such a detection system may be used in, for example, an infrared camera or other infrared imaging equipment. The invention also relates to a method of manufacturing such detectors.
Thermal-radiation detection systems are known comprising a group of the infrared detector elements with the modulation means mounted in front of the detector elements. The temperatures of the detector elements change in response to the incident thermal-radiation. Modulating the transmission of this radiation to the detector elements is particularly important for pyroelectric elements, because the pyroelectric charge is produced only while the temperature of the element is changing. This is normally achieved by chopping the incident radiation at a uniform frequency, the element being exposed to radiation at a reference temperature (from the chopper blade) while transmission of the radiation from the scene being viewed is interrupted by the chopper blade. Thus, the modulation determines a transmission frequency for the incidence of the thermal-radiation on the detector elements.
In general, pyroelectric and ferroelectric materials are also piezoelectric. Hence the detector elements also produce electrical output if subjected to varying stress (for example by vibration or shock). This phenomenon, termed microphony, constitutes undesired background noise which interferes with the detection of the thermal radiation. In order to reduce microphony, it is known to mount the detector elements on a first face of a flexible substrate of thermally insulating polymer material (such as a very thin polyimide membrane) having first and second opposite faces. The flexibility of this known substrate derives from the thinness of the membrane, because polyimide in itself is not a very flexible material. Such a mounting arrangement is described in, for example published European patent application EP-A-0 041 297 , published UK patent application GB-A-2 100 058 and the paper entitled "The application of heat-collector fins to reticulated pyroelectric arrays" by A. A. Turnbull, presented in Cannes, France in November 1985 and published in the Proceedings of SPIE (Society of Photo-Optical Instrumentation Engineers, USA) Vol 588, Recent developments in materials and detectors for the infrared (1985), pages 38 to 43. The whole contents of these two patent applications and the said 1985 SPIE paper are hereby incorporated as reference material in the present specification.
It is known from said 1985 SPIE paper to manufacture such a device by mounting a body of the pyroelectric material (for example ceramic PLMZT, lead lanthanum manganese zirconium titanate) on a substrate, sawing grooves through the thickness of the body to reticulate the material for the individual detector elements, transferring the whole reticulated array in one operation from the substrate to the polyimide membrane which carries the bottom electrode connection pattern, and thereafter providing a top electrode connection (for example carried by a second polyimide membrane which preferably also carries a heat-collector fin). In this manner a high performance pyroelectric detector element array is formed with good thermal isolation and low microphony and in a comparatively inexpensive manner.